1. Field of the Invention
The invention relates to a tool in accordance with the preamble of claim 1.
2. Description of the Related Art
Tools, in particular hand-held tools, such as e.g. a rammer for ground compaction purposes, which comprise a combustion engine as the drive are known in various forms. Two-cycle engines have proven to be particularly suitable as combustion engines and have a high specific output and are not dependent upon position when using suitable carburettors so that they can even be used in a steeply inclined position.
In essence, two principles are known for the lubrication of two-cycle engines, namely petroil lubrication, in which oil is already admixed beforehand to the fuel at a mixing ratio of 1:25 to 1:100, and separate-lubrication, in which oil is pumped from a separate oil tank by means of an oil pump into the crank housing or the carburettor nozzle. Both lubricating methods help make it possible to utilise two-cycle engines in any position and also serve to keep the weight of such engines low.
Whereas during petroil lubrication the oil must be added to the fuel during each fuelling procedure, it is necessary in the separate-lubricating process to top up the separate oil tank at regular intervals, as in the case of two-cycle engines there is basically a loss lubrication, i.e. there is no oil circulation. Apart from the weight of the additional oil tank, the requirement for regular cleaning and also the provision of an oil tank indication, safety devices are required which prevent the engine from starting-up if oil supplies are not sufficient. These devices increase considerably the complexity and the weight of the essentially simple and robust two-cycle engine.
DE 198 00 904 A1 discloses a combustion engine for a tool, wherein a cam drive for a valve control of the engine is lubricated by means of an oil sump. The rotational movement of the toothed wheels which are associated with the cam drive serves to generate an oil mist which can be guided into a crank housing of the engine where it lubricates the moving parts.
It is the object of the invention to provide a tool having a lubricating system for the combustion engine, wherein the complexity is reduced with respect to lubricating systems which are known from the prior art.
The tool is characterised in that the combustion engine can be lubricated by the oil from the housing.
With respect to the invention, it is assumed that the tool is already provided with a large quantity of oil, namely for the purpose of lubricating the movement-conversion device, i.e. the transmission in the housing. In accordance with the invention, this oil is then also used for the purpose of lubricating the combustion engine. The oil from the transmission is consequently consumed over the course of time. An additional oil tank as in the case of the known separate-lubricating procedure is thus not required nor is it necessary to fill the tool with an oil-fuel mixture.
In an advantageous manner, the combustion engine can be lubricated exclusively with the oil from the housing and the oil quantity provided in the housing is such that it is sufficient for the purpose of lubricating the movement-conversion device and the combustion engine during a predefined servicing interval. This relieves the operator of the obligation of checking the oil level at regular intervals, of topping up the oil or supplying a mixture. On the contrary, topping up the oil into the housing which accommodates the movement-conversion device is performed by a specialist within the scope of a service inspection which must be carried out at regular intervals in any case for such highly stressed tools.
In an advantageous manner, a conveying device is provided for the purpose of conveying oil from the housing, which surrounds the movement-conversion device, to the combustion engine.
Since the quantity of oil present is thus limited, it is necessary to use the oil extremely economically during lubrication of the oil-consuming combustion engine.
Thus, in the case of a particularly advantageous embodiment of the invention, the oil which is supplied to the combustion engine is provided in the form of an oil aerosol, i.e. as oil mist. The oil aerosol is generated in the housing by means of the movement-conversion device, in which numerous components move at high speed, whereby the oil provided in a sump is extensively centrifuged in the form of droplets and very finely atomised.
In the case of an advantageous development of the invention, the oil or oil aerosol is collected by means of a collecting device, which is coupled to the conveying device, in the housing. In order to assist the collecting procedure, it is expedient if the conveying device comprises a suction device. In order to be able to transport the oil to the combustion engine, it is also advantageous if the conveying device comprises a pressure-generating device for the purpose of generating a pressure gradient between the housing-side and the combustion engine-side. The pressure-generating device can also serve to pressurise the oil or the oil aerosol and to inject it at the combustion engine.
In the case of a particularly advantageous embodiment of the invention, the pressure-generating device comprises a pressure reservoir which is coupled to a combustion chamber of the combustion engine. Upon ignition of the air-fuel mixture, there is a sudden increase in the pressure in the combustion engine, wherein some of the high pressure passes in the form of pressurised exhaust gas from the combustion chamber into the pressure reservoir.
As already discussed above, the oil or oil aerosol provided must be used extremely sparingly, so that the oil supply is sufficient over the servicing interval. It has proven to be particularly advantageous, if the oil or the oil aerosol can be discharged in a region of a contact surface between a piston and a cylinder of the combustion engine. The contact surface is the region of the combustion engine which in tribological terms is loaded to the greatest extent and is thus in greatest need of lubrication.
In the case of a particularly advantageous embodiment of the invention, there is provided an oil outlet which is fed by the conveying device and through which the oil or oil aerosol can be discharged, depending upon the position of the piston in the cylinder, on to a running surface of the piston shaft and on to a running surface of the cylinder lying opposite the oil outlet. As a consequence, the contact surface between the piston and the cylinder can be wetted in a controlled manner with oil or the oil aerosol.
The conveying device which guides the oil from the housing to the combustion engine can be controlled in an advantageous manner in dependence upon the operating state of the engine. For example, it has been shown that the combustion engine does not require lubrication in the no-load state, whereas in the full-load state there is an increased requirement for lubrication. The controllability of the conveying device again considerably enhances the ways of utilising the limited oil supplies economically.